High conductivity electrolyte gel materials

ABSTRACT

An electro-optical device is provided which is useful in control of visible and infrared absorption by windows, data display devices and the like typically comprising in sandwich arrangement a pair of electrodes, and disposed therebetween, two identical layers of transition metal electrochromic compounds separated by a semi-solid highly conductive sulfuric acid gel electrolyte. The gel exhibits good conductivity, stability and compatibility with the electrochromic layers.

OR 3t708a220 United S616:

Meyers et a1.

[54] HIGH CONDUCTIVITY ELECTROLYTE GEL MATERIALS [75] Inventors: MarionDonghs Meyers, Stamford; Tholnu Anthony Augurt, Bridgeport, both ofConn.

[73] Assignee: American Cyanlnid Company, Stamford, Conn.

[22] Filed: May 25, 1970 [211' Appl. No.: 41,155

[52] US. Cl. ..350/160 R [51] Int. Cl. ..G02i 1/28 [58] FieldotSeareh..350/160 [56] References Cited I UNITED STATES PATENTS 3,443,859 5/1969Rogers .350/160 3,451,741 6/1969 Manos..... .....350/160 111 3,708,220[451 Jan. 2, 1973 3,453,038 7/1969 Kim e! I]. ..350/1 60 3,521,9417/1970 ..350/l60 3,303,488 2/1967 Anderson ..350/l60 3,153,113 10/1964Flanagan et a1. ..3$0/160 Primary Examiner-Ronald L. Wibert AssistantExaminer-J. Rothenberg Attorney-Charles J. Fickey [57] ABSTRACT 7Claims, 7 Drawing Figures PATENTEDJAN 2 I975 SHEET 2 [IF 3 ACT/ VE AREA\NES'A 47 A/E'SA 42 THOMAS ANTHONY AUGURT HIGH CONDUCTIVITY ELECTROLYTEGEL MATERIALS This invention is directed to electro-optical deviceswhose electromagnetic radiation transmission characteristics can beselectively altered by the influence of a suitably controlled electricalfield. More particularly this device relates to a system comprisingelectrochromic materials and conductive electrolyte which exhibit goodchemical and electrochemical stability, reversibility andreproducibility. Still more particularly this invention is concernedwith a highly conductive sulfuric acid gel electrolyte which exhibitsgood chemical compatibility with the electrochromic materials and a widerange of desirable gel properties.

In prior copending commonly assigned U.S. applications, Ser. No.530,086, filed Feb. 25, 1966, refiled as Ser. No. 616,791 Feb. l4, 1967,refiled as Ser. No. ll0,068 Jan. 27, 1971, and Ser. No. 534,188, filedMar. 14, 1966, refiled as Ser. No. 616,790 Feb. 17, 1967, refiled asSer. No. 120,365 Mar. 2, l97l, and continuation-in-part applicationsthereof, there are described electro-optical devices exhibiting aphenomenon known as persistent electrochromism.

The term persistent electrochromism denotes the property of a materialwhereby its electromagnetic radiation absorption characteristic isaltered, in most instances even at ambient temperature, under theinfluence of an electric field. Such materials, for example, may exhibitlittle or no absorption of visible wave lengths in the absence of anelectric field and therefore be transparent, but when subjected to anelectric field, effectively absorb in the red end of the spectrum,turning blue in color. Similar effects can be observed in other portionsof the electromagnetic radiation spectrum, invisible as well as visible.

As described in the foregoing earlier applications, if

a layer of a persistent electrochromic material is disposed between apair of electrodes, across which a potential is applied, the radiationtransmitting characteristic of the material will change. if theelectrodes and the electrochromic layer are formed on the surface of atransparent substrate, such as glass, the light transmittingcharacteristics of the combination can be varied by controlling theelectric field produced across the electrochromic material. On thesubstrate which originally is clear, i.e., presenting substantially nodiminution of the light transmitting ability of the substrate,application of a voltage between the electrodes to establish an electricfield of the proper polarity changes the light absorption characteristicof the electrochromic material, turning it darker, for example, thusdecreasing the light transmitting ability of the entire assembly.

In U.S. Pat. No. 3,357,930 it has been proposed to form electricallyconductive transparent coatings from combination of salts in a polymericmatrix. However,

conducting media such as liquid sulfuric acid, or lithium stearategreases. Although these devices are effec-- tive to change theirelectromagnetic radiation transmitting properties under the influence ofan electric field the simple combination of electrodes, electrochromicmaterials and electrolyte was somewhat limited in application because insome cases the semi solid conducting greases were insufficientlyelectrically conductive to permit device operation at voltages below thedecomposition potential of the electrolyte to avoid undesirableelectrochemical side reactions.

Similarly the use of a strong liquid sulfuric acid elec-- trolyte whilesufficiently conductive to permit low voltage operation was however notchemically compatible to prevent chemical attack on certain electrodematerials such as porous lead, moreover, the liquid electrolyte had thedisadvantages of containment inherent in a The shortcomings describedabove have been overcome in the present invention by the unexpectedstability of certain gelled, sulfuric acid polymeric electrolytes suchas H,SO PVA (polyvinyl alcohol) which possess high electricalconductivity and good chemical compatibility with the electrochromicfilms employed in the present device. In addition these gel electrolytespossess good stability, high viscosity and transparency. Further, theease of device manufacture is facilitated by improved dimensionalstability and self sealing characteristic of the sulfuric acid.polyvinyl alcohol gel employed.

SUMMARY OF THE INVENTION An object of the present invention therefore isto provide an improved form of electrochromic device wherein a highlyconductive gel electrolyte is employed which is chemically stable andcompatible with electrochromic films so as to permit highly stable,reversible and reproducible operation at low potential.

Briefly stated the present invention modifies the priorelectrolyte-electrochromic material sandwich by substituting a stableand highly conductive semi solid sulfuric acid-polyvinyl alcohol gelbetween electrochromic films of the same material. It has been foundthat when such a material is added to the prior device not only does itexhibit markedly improved ionic conductivity with a semi-rigidconducting media but also shows unexpected chemical and electrochemicaladvantages for the electrochromic material-conductive gel combination.in certain cases the properties of the gel may be varied by using othergrades of polyvinyl alcohol, different sulfuric acid concentrations anddifferent polyvinyl alcohol to acid ratios.

The stability and conductivity of the gel, its compatibility with theelectrochromic material and the lower voltage at which the colorationand bleaching can be effected are improved in relation to theembodiments of the earlier disclosures, Ser. No. 41,154 and 41,153. Thusthe present invention is particularly applicable to a much wider rangeof use such as to mirrors, windows, data displays and the like.Moreover, the field of practical use is widened by the ability to varythe gel properties over a wide range of desirable characteristics.

The foregoing and other features, objects and advantages of the presentinvention will become more apparent from he following detaileddescription.

As used herein, a persistent electrochromic material" is defined as amaterial responsive to the application of an electric field of a givenpolarity to change from a first persistent state in which it isessentially non-absorptive of electromagnetic radiation in a given wavelength region, to a second persistent state in which it is absorptive ofelectromagnetic radiation in the given wave length region, and once insaid second state, is responsive to the application of an electric fieldof the opposite polarity to return to its first state. Certain of suchmaterials can also be responsive to a short circuiting condition, in theabsence of an electric field, so as to return to the initial state.

By persistent is meant the ability of the material to remain in theabsorptive state to which it is changed, after removal of the electricfield, as distinguished from a substantially instantaneous reversion tothe initial state, as in the case of the Franz-Keldysh effect.

DETAILED DESCRIPTION OF THE INVENTION- Electrochromic Materials Thematerials which form the electrochromic materials of the device ingeneral are electrical insulators or semi-conductors. Thus are excludedthose metals, metal alloys, and other metal-containing compounds whichare relatively good electrical conductors.

While not wholly understood, it appears that coloration of theelectrochromic materials must be accompanied by the uptake of positivecounterions provided in the electrolyte.

The persistent electrochromic materials are further characterized asinorganic substances which are solid under the conditions of use,whether as pure elements, alloys, or chemical compounds, containing atleast one element of the Periodic System which can exist in more thanone oxidation state in addition to zero. The term oxidation state asemployed herein is defined in Inorganic Chemistry, T. Moeller, JohnWiley & Sons, Inc., New York, 1952. These include materials containing atransition metal element (including Lanthanide and Actinide serieselements), and materials containing non-alkali metal elements such ascopper. Preferred materials of this class are films of 'transition metalcompounds in which the transition metal may exist in any oxidation statefrom +2 to +8. Examples of these are: transition metal oxides,transition metal oxysulfides, transition metal halides, selenides,tellurides, chromates, molybdates, tungstates, vanadates, niobates,tantalates, titanates, stannates, and the like. Particularly preferredare films of metal stannates, oxides and sulfide of the metals of Groups(lV)B, (V)B and (VI)B of the Periodic System, and Lanthanide seriesmetal oxides and sulfides. Examples of such are copper stannate,tungsten oxide, cerium oxide, cobalt tungstate, metal molybdates, metaltitanates, metal niobates, and the like.

Additional examples of such compounds are the following oxides: MOoxides (M representing the metal ion), e.g., MnO, NiO, CoO, etc.; M,O,oxides, e.g., C130,, F610;, Y,O,, Ybgoa, V 0 Tl,O;, Mn,0,, CRL; MO,oxides, e.g., TiO,, NnO,, ThO,, CrO,, etc.; M 0 oxides, e.g., Co,0,,Mn,0,, Fe,0,, etc.; MO, oxides, e.g., CrO,, U0 etc.; M,O oxides, e.g.,V,O,, Nbgos, Ta,0,, etc.; Mp. oxides; M,O oxides such as M,O complexoxides such as those of the formula XYO, (X

and Y being different metals), e.g., LiNiO,, etc.; XYO, oxides, e.g.,LiMnO,, FeTio MnTiO CoTiO,, NiTi0,, LiNb0,, LiTaO,, Nawo, etc.; XYO,oxides, e.g., MgWO CdWo,, NiWO,, etc.; XY,O,, e.g., CaNb,O, (Niobite"oxides); X,Y,O,, e.g., Na,Nb,O.: Spinel structure oxides, i.e., of theformula X,YO e.g., NaMoO Ag,MoO Cu,MoO,, Li,MoO,, Li,WO,, Sr,TiO,,Ca,MnO etc.: and XY,O,, e.g., FeCr,O,, TiZn,O etc.; X,YO oxides, e.g.,Fe,TiO,, Al TiO etc. For a discussion of some complex oxides, seeAdvanced Inorganic Chemistry, Cotton & Wilkinson, p. 51, (I966),Interscience Publishers, Inc., New York and Progress in Inorganic Chem.,Vol. l, 465 (I959) lnterscience Publishers, Inc., New York. Alsoincluded are nitrides, and the sulfides corresponding to the aboveoxides. Hydrates of certain metal oxides may also be used, e.g., WO.I-I,O, WO,.2I-I,O, MoO -.H,O and MoO .2H,O.

The preferred electrochromic material for use with the semi-solidelectrolyte of the present invention is a compound which contains atleast one element selected from Group VA, VIA, VIIA of the PeriodicTable of the elements and at least one cation from Groups IB, IIB toVIIIB including Lanthanide and Actinide series. Particularlyadvantageous materials are W0;, and M00,.

An important advantage of devices of the invention containing a stablesulfuric acid gel electrolyte in contact with electrochromic material isapplicability to large uniformly colored areas. The invention,therefore, permits numerous practical applications where control ofvisible and infrared absorption is desired to which prior artelectrooptical devices are not susceptible as for example windows inhomes, commercial buildings and automobiles.

When the persistent electrochromic materials are employed as films,thickness desirably will be in the range of from about 0. 1-100 microns.However, since a small potential will provide an enormous field strengthacross very thin films, the latter, i.e., 0.ll0 microns, are preferredover thicker ones. Optimum thickness will also be determined by thenature of the particular compound being laid down as films and by thefilmforming method since the particular compound and film-forming methodmay place physical (e.g., nonuniform film surface) and economiclimitations on manufacture of the devices.

The films may be laid down on any substrate which, relative to the film,is electrically non-conducting. Suitable substrate materials includeglass, wood, paper,- plastics, plaster, and the like, includingtransparent, translucent, opaque or other optical quality materials. Apreferred embodiment in the instant device would employ at least onetransparent electrode.

When the electric field is applied between the electrodes, a bluecoloration of the previously transparent sandwich occurs, i.e., thepersistent electrochromic layer becomes absorptive of electromagneticradiation over a band encompassing the red end of the visible spectrum,thereby rendering it bluish in appearance.

Prior tothe application of the electric field, it was essentiallynon-absorbent and thus transparent.

ELECTROLYTE A semi-solid conductive electrolyte gel is employed. Theelectrolyte comprises in combination with sulfuric acid a gellingmaterial for the acid. Any gelling agent which is compatible with theelectrochromic layer is suitable. Particularly advantageous gellingagents are polyvinyl alcohol, polyacrylamide, ethylene glycol, sodiumsilicate, cabo-sil, and the like.

A thickening agent such as Purifloc A21 may optionally be employed.

A preferred embodiment employs H,SO in combination with polyvinylalcohol. The properties of this gel may be varied in advantageous mannerby employing polyvinyl alcohol of various molecular weights, differingsulfuric acid concentration and different polyvinyl alcohol to acidratios. Thereby gel electrolytes can be produced to give a specificconductivity in the range of from about 0.10 to 0.60 ohm cm.

A distinct advantage of the above mentioned gel electrolytes is theirhigh ionic conductivity and good chemical stability. We have found thatboth requirements are unexpectedly met by gels in the preferredconductivity range of 0.20 0.40 ohm cm".

Other materials may be incorporated into the gel to vary the physicalproperties of the gel such as viscosity and vapor pressure. Thus thecomposition may optionally include organic solvents such as, dimethylformamide, acetonitrile, proprionitrile, butyrolactone and glycerine.

Further, the gels used in the instant invention may be made opaque with,for example, polyhydric phenols such as gallic acid for use in certainelectrochromic display device applications.

COUNTER ELECTRODE If the cell is to be used as a light transmissionmodulating device, it employs at least one transparent electrode and asecond counter electrode in contact with the solution. The latterelectrode is one selected from a group of materials compatible with theelectrolyte, as previuosly discussed, such as tungsten oxide ormolybdenum oxide. It is advantageous to use the same material for bothelectrodes, although not necessary. In a preferred embodiment of thepresent invention tungsten oxide counter electrode is employed incontact with the electrolyte.

The invention may be better understood by reference to the drawingswhich show embodiments of the invention.

FIG. 1 is a cross-sectional view in part of an electrochromic device ofthe invention, showing the connecting electrical power circuit,

FIG. 2 is a planview of the device of FIG. 1,

FIG. 3 is a cross-sectional view of taken along the lines 3-3 of FIG. 2,showing the complete device of FIG. 1,

FIG. 4 is a view of an alternative embodiment of the inventiveelectrochromic device having a planar counter-electrode,

FIG. 5 is a cross-sectional view taken along the lines 5-5 of FIG. 4,

FIG. 6 is a partial cross-sectional view of an alternative embodimentwith a grid counter-electrode,

FIG. 7 is a partial cross-sectional of a device view with a counterelectrode in the border of the device.

With reference to FIG. 1, I0 is a substrate or backing material overlaidwith a first electrode 11. The substrate l0 and the conductive layer 11may conveniently be provided as a unit by so-called NESA-g lass, acommercially available product having a transparent coating ofconductive tin oxide layer 11, deposited on glass. Onthe NESA glass, isdeposited a layer of a persistent electrochromic material I2 for exampleby vacuum evaporating tungsten oxide to form a film 12 of thickness inthe range of about l micron. Overlaying and in contact with film 12 is agasket 13 formed from an electrically insulating material. As shown inFIGS. 2 and 3 the gasket 13 separates the film 12 from a glass coverplate 14 and produces a space for the gel electrolyte 15. In the end ofthe electrolyte chamber is a tungsten oxide counter electrode 16.

Contact to an external potential is secured by lead 18 from a conductivestrip 19 over the tin oxide layer 11 of the conducting glass, and lead20 to counter electrode 16. A source of dc potential 30 is coupledbetween the conductive films with its positive terminal connected to themetallic strip 19 and its negative terminal connected to the tungstenoxide counter electrode 16.

Turning now to drawing, FIG. 4 a plan view of another arrangement of thedevice of the present invention showing an electrochromically activecenter area and the external electrical leads is illustrated.

The components of the sandwich device are shown in cross section in FIG.5. The outermost layers 42 and 47 consist of two pieces of transparentsubstrate containing on their inner surface an electrically conductivecoating. The substrate and conductive layer may conveniently be providedas a unit by so-called NESA" glass, a commercially available producthaving a transparent coating of conductive tin oxide on one surface ofthe glass as described previously. Deposed thereon and in contact witheach conductive tin oxide layer are separate evaporated films oftungsten oxide 43 and 46 each about 1 micron in thickness. Between andin contact with both tungsten oxide films there is disposed a centerportion of an opacified gel 45 about 3 mm in thickness enclosed by acircular insulating spacer 44. Electrical contact of the device to theexternal potential is accomplished by soldered electrical wireconnections 48 and 49 from the NESA conductive layers. The electricalpathway through the cell is from one tungsten oxide layer, through theconductive electrolyte gel to the other tungsten oxide layer. Because ofthe arrangement of the two electrochromic films in opposed relationshipthe arrangement shown in FIGS. 3 and 4 permits the display of only oneelectrochromic layer at a time.

FIG. 6 shows a geometric arrangement of the device applicable as awindow wherein one electrochromic layer 50 is fabricated in the form ofa grid with a high percent transmission even when the material is in thecolored state.

Alternatively FIG. 7 shows an embodiment wherein the electrochromiccounter electrode layer I6 is concealed in the frame of a window so asnot to interfere with optical transmission.

The following examples illustrating particular applications of thepresent invention are not to be construed .as a limitation of theinvention except as defined in the appended claims.

EXAMPLE 1 A film of tungsten oxide about 1.0 micron in thickness isvacuum deposited on two electronically conductive NESA glass plates. Theplates were assembled in a sandwich arrangement in which the tungstenoxide layers were positioned opposite and in face to face arrangement ina cell cavity provided by a 2 mm spacer. A suitably conductive gel wasprovided by sulfuric acid polyvinyl alcohol. The gel preparationcomprises the addition of 20 ml of 3M l-l,SO to 5.5 gm of polyvinylalcohol (DuPont Elvanol 70-05) at a temperature of 80 C. A clear viscoussolution is formed within about minutes. During this period the mixtureis stirred constantly and the temperature is held at about 80-85 C. Thesolution is cooled to room temperature while centrifuging to removebubbles.

The clear liquid is poured into the empty cell cavity through one portwhile providing an additional port as an outlet for the displaced air.The gel exposed at the entrance port hardens in contact with air therebyproviding self seal and antileak features.

The gel exhibited a conductivity of 0.30 ohm cm and unexpected clarityon storage. The cell underwent 5,000 cycles over a period of days. Thefilm remained transparent and the device showed no change in voltagetime curves.

EXAMPLE 2 Gel Gel Conductivity Number Composition ohm cm Voltage Cell(Volts) Cycles 3 3M H.80

30% Elvanol 0.l9 0.8 5000 -05 4 4M H,SO.

13% PVA 125 0.33 0.7 l0,000 5 5M H,SO.

13% PVA I25 0.37 0.8 7500 6 8M H,S0.

40% Elvanol 0.25 L0 8000 In the foregoing examples all cells were inoriginal condition after the number of cycles specified.

We claim:

1. A variable light transmission device comprising a layer of apersistent electrochromic material, said material having its opticalabsorption alterable by addition or removal of electrons, in contactwith a layer of a semi-solid electrolyte gel, said gel andelectrochromic material being between a pair of electrodes.

2. A variable light transmission device, as in claim 1, which comprisesa laminate of at least two layers of said persistent electrochromicmaterial separated by a semi solid electrolyte gel disposed between saidpair of conductive electrodes.

3. The device of claim 1 wherein said semi-solid electrolyte is asulfuric acid-polyvinyl alcohol gel.

4. The device of claim 1 wherein said gel electrolyte has a conductanceof at least that of 4M H 5. The device of claim 2 wherein said gel is mface to face electrical contact with 'two separate but identical filmsof tungsten oxide.

6. The device of claim 2 wherein at least one of said electrodes issubstantially transparent.

7. The device of claim 1 wherein said persistent electrochromic materialis tungsten oxide.

t t Il

2. A variable light transmission device, as in claim 1, which comprisesa laminate of at least two layers of said persistent electrochromicmaterial separated by a semi solid electrolyte gel disposed between saidpair of conductive electrodes.
 3. The device of claim 1 wherein saidsemi-solid electrolyte is a sulfuric acid-polyvinyl alcohol gel.
 4. Thedevice of claim 1 wherein said gel electrolyte has a conductance of atleast that of 4M H2SO4.
 5. The device of claim 2 wherein said gel is inface to face electrical contact with two separate but identical films oftungsten oxide.
 6. The device of claim 2 wherein at least one of saidelectrodes is substantially transparent.
 7. The device of claim 1wherein said persistent electrochromic material is tungsten oxide.